Resistance variable memory elements, which include Programmable Conductive Random Access Memory (PCRAM) elements using chalcogenides, have been investigated for suitability as semi-volatile and non-volatile random access memory devices. A typical chalcogenide resistance variable memory element is disclosed in U.S. Pat. No. 6,348,365 to Moore and Gilton.
In a typical chalcogenide resistance variable memory element, a conductive material, for example, silver, tin and copper, is incorporated into a chalcogenide glass. The resistance of the chalcogenide glass can be programmed to stable higher resistance and lower resistance states. An unprogrammed chalcogenide variable resistance element is normally in a higher resistance state. A write operation programs the element to a lower resistance state by applying a voltage potential across the chalcogenide glass and forming a conductive pathway. The element may then be read by applying a voltage pulse of a lesser magnitude than required to program it; the resistance across the memory device is then sensed as higher or lower to define two logic states.
The programmed lower resistance state of a chalcogenide variable resistance element can remain intact for an indefinite period, typically ranging from hours to weeks, after the voltage potentials are removed; however, some refreshing may be useful. The element can be returned to its higher resistance state by applying a reverse voltage potential of about the same order of magnitude as used to write the device to the lower resistance state. Again, the higher resistance state is maintained in a semi- or non-volatile manner once the voltage potential is removed. In this way, such an element can function as a semi- or non-volatile variable resistance memory having at least two resistance states, which can define two respective logic states, i.e., at least a bit of data.
One exemplary chalcogenide resistance variable device uses a germanium selenide (i.e., GexSe100−x) chalcogenide glass as a backbone. The germanium selenide glass has, in the prior art, incorporated silver (Ag) and silver selenide (Ag2+/−xSe) layers in the memory element. FIG. 1 depicts an example of a conventional chalcogenide variable resistance element 1. A semiconductive substrate 10, such as a silicon wafer, supports the memory element 1. Over the substrate 10 is an insulating material 11, such as silicon dioxide. A conductive material 12, such as tungsten, is formed over insulating material 11. Conductive material 12 functions as a first electrode for the element 1. An insulating material, 13 such as silicon nitride, is formed over conductive material 12. A glass material 51, such as Ge3Se7, is formed within via 22.
A metal material 41, such as silver, is formed over glass material 51. An irradiation process and/or thermal process are used to cause diffusion of metal ions into the glass material 51. A second conductive electrode 61 is formed over dielectric material 13 and metal material 41.
The element 1 is programmed by applying a sufficient voltage across the electrodes 12, 61 to cause the formation of a conductive path between the two electrodes 12, 61, by virtue of a conductor (i.e., such as silver) that is present in metal ion laced glass layer 51. In the illustrated example, with the programming voltage applied across the electrodes 12, 61, the conductive pathway forms from electrode 12 towards electrode 61.
A plurality of resistance variable memory elements can be included in a memory array. In doing so, it is desirable to provide a high density of memory elements.